The present invention relates generally to cathode-ray storage tubes, and more particularly to an improved storage target structure for such tubes.
U.S. Pat. No. 3,293,474, issued Dec. 20, 1966 to Charles B. Gibson, Jr. and assigned to the assignee of the present invention, discloses a storage target construction that includes a mesh-like collector electrode formed on one side of an insulating support plate. Deposits of a bistable storage material, suitably a phosphor of the P-1 type, are disposed in the openings of the electrode and isolated from one another by the mesh-like conductor.
In the commercially produced embodiment of the Gibson, Jr. target--i.e., the one shown in FIG. 2 of the above-mentioned patent--the deposits of phosphor storage material physically contact the conductive mesh. This causes an undesirable effect known as "rim-lighting" during operation of the target in a storage CRT. Rim-lighting is characterized by a halo or ring of light surrounding storage phosphor deposits in the unwritten or background areas of the target. The resulting increase in background luminance significantly reduces display contrast and impairs the viewability of displayed information.
Several explanations for rim lighting have been advanced. One is that the surface of each unwritten phosphor deposit somewhat attains a surface charge that decreases with distance from the deposit's interface with the collector electrode. (As is understood, the collector normally is maintained at a potential several hundred volts higher than the CRT's flood gun cathodes.) At some distance D from the interface, the deposit's surface potential is equal to the first crossover voltage for the phosphor, i.e., the voltage at which its secondary emission ratio .delta. is unity. The surface potential for regions of the phosphor lying between the collector interface and point D thus will be above first crossover (.delta.&lt;1), and flood gun electrons will tend to charge those regions in a positive direction--i.e., toward collector potential. However, at distances greater than D from the collector/phosphor interface, the surface of the deposit will be below first crossover (.delta.&lt;1) and those regions will be charged in the opposite direction toward flood gun cathode potential--typically 0 volts. As a result, each phosphor deposit will include a positively charged peripheral region that remains in a luminous or "written" state whenever the CRT is operated in a storage mode.
It might seem that rim-lighting could be avoided by making the phosphor deposits slightly smaller than the mesh apertures so that they do not contact the collector electrode, but this has not proved to be effective unless a relatively large separation is created. Thus, in the version of the Gibson, Jr. target shown in FIG. 4 of the patent, rim-lighting would be observed unless the depth of cavities 66 was significantly increased or the thickness of the phosphor storage dielectric layer 36' was reduced a corresponding amount. Both alternatives are unattractive, particularly from a manufacturing standpoint.
The problem of rim-lighting has been addressed in a different type of storage target--one of the raised "dot" collector type--by providing each conductive dot with a collar of insulating material. Thus, as shown in U.S. Pat. No. 4,159,439, issued June 26, 1979 to Duane A. Haven et al. and assigned to the assignee of the present invention, all but the outermost end of each raised collector member is coated with an insulating material to isolate the collector structure from the surrounding phosphor storage layer. While generally effective in minimizing rim-lighting, the process used to apply the insulating collars is not well suited for use in commercial manufacturing operations.
Storage targets of the type described above are generally erased by applying a negative pulse to the collector structure. Capacitive coupling between the collector and storage phosphor drops the written areas of the target below the first crossover, after which the flood gun electrons complete the erasure. The collector voltage is then raised to normal operating potential at a rate slow enough for flood gun action to hold the storage phosphor below first crossover. As will be understood, the erasure process affects all areas of the target having a common collector electrode--usually the entire target or a major portion of it. This is a significant drawback in certain applications, since there is no way to erase a selected portion of a stored image without also easing other information one may wish to retain.
U.S. Pat. No. 3,611,000, issued Oct. 5, 1971 to Kent H. Johnson and assigned to the assignee of the present invention, describes a method of selectively erasing a bistable storage target that includes a layer of storage phosphor disposed atop a target electrode, and a separate collector electrode in contact with the phosphor layer on the side opposite the target electrode. Targets having such a structure are the subject of U.S. Pat. Nos. 3,594,607, issued July 20, 1971 to Roger A. Frankland and 4,185,227, issued Jan. 22, 1980 to Robert W. Morris, both assigned to the assignee of the present invention. A storage target having separate coplanar interdigitated target and collector electrodes for selective erasure is described in U.S. Pat. No. 4,139,800, issued Feb. 13, 1979 to Bruce H. Ostermier et al. and also assigned to the assignee of the present invention. The Frankland target is difficult to manufacture because the storage layer must be highly uniform in thickness, and the collector electrode is produced by vacuum deposition through a mask. The phosphor deposits of the Morris and Ostermier et al. targets are in contact (or near-contact) with their collector electrodes, making them subject to rim-lighting.
The present invention has as its principal object the provision of improved storage targets of the type that include an apertured web collector electrode.
A more specific object of the invention is to provide an improved storage target of the Gibson, Jr. type having minimal background luminance.
A still more specific object of the invention is to provide a storage target of the type that includes an apertured web collector and a discontinuous layer of secondary emissive material formed by a plurality of phosphor deposits disposed within the web apertures, wherein a layer of an insulating material overlying the collector web isolates the phosphor deposits to minimize rim-lighting.
Another object of the invention is to provide an insulated web collector storage target capable of selective erasure of stored information.
A related object of the invention is to provide an insulated web collector storage target structure that includes separate, electrically isolated target and collector electrodes.
A further object of the invention is to provide an improved insulated web collector storage target capable of displaying stored information in a color that contrasts with that of the background.
Still a further object of the invention is to provide an improved insulated web collector storage target capable of displaying stored information in a color that differs from that of displayed but unstored (i.e., "write-through") information.